Cannot set a compose key anymore

Submitted by Jehan @Jehan

Description

I just updated to Fedora 26 and am up-to-date.
gnome-tweak-tool 3.24.1-2.fc26
gnome-shell 3.24.3-2.fc26

Using the same config as previously, but I lost my compose key capability (on CapsLock). Checking the tweak tool GUI, it is still checked though. I tried to set it on Right Alt, or Right Ctrl as well but no more luck. Looks like the Compose key feature is broken in GNOME Tweak Tool 3.24 (unless it is broken at a lower level). ;-(

Steps:

Open GNOME Tweak Tool

Go to tab "Typing"

Under "Position of Compose key", set to "Caps Lock"

Under "Caps Lock behavior", make sure it is still "Disabled"

Expected result: Caps Lock should now be able to be used as a Compose key. For instance holding CapsLock+e+` should input "è".